A typical thin film integrated circuit resistor consists of a resistive block of polysilicon with a low sheet resistance (e.g., less than 1000 ohms per square). The resistive block is fabricated above a major surface of a semiconductor substrate, along with other components of an integrated circuit, and separated from the substrate by a passivation layer. The substrate typically is doped to be conductive, and it is connected to either the most positive or the most negative potential, depending on whether it was doped with p-type or n-type impurities, respectively
The typical thin film integrated circuit resistor is substantially linear for applied voltages less than 100 volts. For applied voltages greater than 100 volts, however, the typical resistor is subject to breakdown, and to nonlinearity. The nonlinearity is due to effects on the charge carrier density within the resistive block. The charge carrier density is affected by electric fields associated with potential differences between regions of the resistive block and the substrate (substrate bias voltages).
In some high voltage applications, the linearity of a thin film integrated circuit resistor is not important. Resistor linearity, however, is of great consequence in analog applications, such as high voltage sensors, precision current to voltage converters, and digital to analog converters. Moreover, resistor linearity is a particular concern in analog applications that use resistive blocks having a relatively high sheet resistance (e.g., over 1000 ohms per square), since the effects of a given substrate bias voltage electric field on charge carrier density with increasing sheet resistance.
There is therefore a need for a thin film integrated circuit resistor that is substantially linear at applied voltages greater than 100 volts.